1. Field of the Invention
This invention relates generally to electronic tuning apparatus, and is directed more particularly to a variable tuning signal generating circuit using a memory circuit.
2. Description of the Prior Art
In the prior art, it is known to use a storage means such as a digital memory circuit instead of a plurality of potentiometers for generating a tuning signal for an electronic tuner. However, in prior art systems, a memory circuit which has the ability to store a large number of predetermined tuning values is needed. For example, 50 predetermined tuning values must be stored in order to tune in the 50 UHF television channels available in Japan.
An example of a prior art tuning apparatus that uses a memory circuit will be described with reference to FIG. 1. For the sake of simplifying the explanation, only the tuning of UHF channels will be described.
In FIG. 1, 1 designates an electronic UHF tuner and 2 a video detecting circuit. The tuner 1 includes a local oscillator (not shown) which contains a variable reactance element, in this case a variable capacitance diode. This variable capacitance element is supplied with a tuning signal, voltage E which corresponds to a channel to be selected. Furthermore, a storage means, digital memory circuit 3 having 50 addresses, is provided. A predetermined tuning value, in the form of a binary code signal corresponding to the tuning voltage E is stored at each address. A channel selector 4 is provided whose output signal is used as the address signal for the memory circuit 3. This channel selector 4 produces a channel selecting signal in binary form corresponding to a selected channel. Thus, when a desired channel, for example, channel 13, is selected the channel selector 4 produces a channel selecting signal, for example, 000001, corresponding to the selected channel, and the memory circuit 3 is addressed by that channel selecting signal. As a result, a code signal, for example, 00000000010110, developed by the memory circuit 3 is fed to a digital-to-analog (D/A) converter 5, which then produces a tuning signal, voltage E, corresponding to the selected channel 13. This tuning voltage E is applied to the variable capacitance element of the local oscillator in the tuner 1. The channel selecting signal from the channel selector 4 is also fed to a channel display device 6, which then displays the selected channel 13.
With such a prior art tuning apparatus, a separate code signal corresponding to the tuning voltage E for each channel (from the channel 13 to channel 62) must be memorized in the memory circuit 3. The presetting operation of correctly determining and setting the binary code signals for each tuning apparatus is undesirably complicated and the storage capacity required by memory circuit 3 is undesirably large. Since the characteristics of the variable capacitance elements used in the local oscillator of the tuner 1 vary from tuner to tuner, the code signals must be determined and set, independently, for each channel in each tuning apparatus which is made. For example, in each Japanese UHF tuning apparatus, code signals corresponding to the tuning voltages E of 50 stations must each be preset in the memory circuit 3.